In many fields of signal processing, it is of interest to include discrete phase shifters in order to change the electrical length of the signal path. These phase shifters are controllable, allowing two or more different lengths of the signal paths to be obtained.
Many fields of use are conceivable for such discrete phase shifters, mainly for applications where the frequency ranges from radio frequencies to microwave frequencies. They may for example be used in signal mixers and digital radio devices.
One particular field of use for phase shifters is active electrically controlled antennas. For such antennas, it is possible to control the antenna radiation pattern, i.e. it is possible to direct the main lobe at a desired angle and to acquire desired side-lobe levels within certain limitations. For example, it could be desired to create a very low degree of antenna coverage at a certain angle as a source of disturbance is present there, at the same time as the main lobe is directed at an angle where a desired receiver or transmitter is present. These antennas are for example used in mobile telephone systems or in radar equipment.
Such an antenna is preferably in the form of an array antenna comprising a number of antenna elements. Each one of these antenna elements in the array antenna is individually fed with a signal having a certain phase relative to the other antenna elements. By varying the relative phase between the antenna elements, the controllable features described above are possible to obtain.
For each antenna element, the signal fed into the array antenna passes a phase shifter before reaching a certain antenna element, where the phase shifter is controlled to constitute a certain signal path length relative to the other antenna elements.
Today, it is common to construct the phase shifters using MMIC (Monolithic Microwave Integrated Circuit) technology. These phase shifters are usually realized using a combination of high-pass and low-pass filters, where these filters create two branches between which the signal can be switched in order to achieve one of two possible relative phase differences between the two branches. This is shown in the prior art FIG. 1a, where a phase shifter 1 has a left-hand branch 2 that comprises a low-pass filter 3 with a first grounded capacitor 4 and second grounded capacitor 5 and a series inductor 6. A right-hand branch 7 of the phase shifter 1 comprises a high-pass filter 8 with a second grounded inductor 9 and a third series capacitor 10 and a fourth series capacitor 11. By means of a first switch 12 and a second switch 13, it is possible to either choose the left-hand branch 2 and the low-pass filter 3 or the right-hand branch 7 with the high-pass filter 8. The switches are connected to a first phase shifter connection point 14 and second phase shifter connection point 15.
In the FIGS. 1b and 1c, phase and amplitude characteristics are shown over frequency. The low-pass filter 3 has the phase characteristics shown with a solid line 16 in FIG. 1b and the amplitude characteristics shown with a solid line 17 in FIG. 1c. The high-pass filter has the phase characteristics shown with a dotted line 18 in FIG. 1b and the amplitude characteristics shown with a dotted line 19 in FIG. 1c. The phases φ (FIG. 1b) and amplitudes A (FIG. 1c) are represented on the y-axis, and the frequency f on the x-axis.
As evident from FIG. 1b, it is possible to acquire a sufficiently constant phase difference D between the filters 3, 8 when the frequency is changed, i.e. the phase difference D is quite broad banded. The value of the phase difference D is possible to adjust by means of the adjustments to the values of the components 4, 5, 6; 9, 10, 11 in the filters 3, 8. The phase performance over frequency is thus not a limiting factor for the function of the phase shifter 1.
As evident from FIG. 1c, however, the pass bands more or less overlap only at a narrow band B. At this narrow band B, the amplitude performance allows the phase shifter to function adequately, but outside the band B the phase shifter performance is not acceptable. The amplitude performance over frequency is thus a limiting factor for the function of the phase shifter 1. This is a problem, since although limited bandwidth of the phase performance may be compensated for by means of calibration methods, this is not feasible for limited bandwidth of the amplitude performance.
As shown in FIG. 2, a number of phase shifter elements 1a, 1b as seen in FIG. 1a, may be serially connected into a cascaded total phase shifter 20, each having a phase difference between the respective branches. This phase length difference is preferably of different values for each phase shifter elements 1a, 1b, allowing a more or less fine tuning of the total phase depending on the number of phase shifter elements in the cascaded total phase shifter 20. The path is chosen for the incoming signal by means of four switches S′, S″, S′″, S″″.
A variation of the phase shifter as described herein is to use so-called reconfigurable filters to realize the phase shift. In this way only one branch is needed, since both a high-pass filter and a low-pass filter can be realized, one at a time, by means of switches. Large components, such as coils, may be re-used, allowing a more efficient use of the layout. Such a configuration is shown in FIG. 3, where a phase shifter element 21 comprises a first grounded capacitor 22 and a second grounded capacitor 23, a third series capacitor 24 and a fourth series capacitor 25 and a coil 26. By means of a first switch 27, second switch 28, third switch 29 and fourth switch 30, either a low-pass filter or a high-pass filter may be realized between a first connection point 31 and second connection point 32. The current switch setting in FIG. 3 provides a low-pass filter.
These filters may also be cascaded as described previously, in order to allow a more or less fine tuning of the phase, depending on the number of cascaded filters constituting the cascaded total phase shifter.
Both the phase shifter configurations as described herein do, however, have some disadvantages. The switches are generally constituted by transistors, PIN-diodes, or other applicable devices such as MEMS (micro electromechanical systems). These switches comprise parasitics. The following describes the switches as being constituted by transistors, but the general principle is applicable for PIN-diodes and other applicable devices.
In FIGS. 4a and 4b, a switch transistor 33 of a known and widely used type is shown. The transistor 33 has a drain 34, a gate 35 and a source 36, and between the drain 34 and the source 36, a parasitic 37, 38 is present. If the transistor 33 is in an OFF-state, as shown in FIG. 4a, i.e. if the switch is open, a parasitic capacitor 37 is present. If the transistor 33 is in an ON-state, as shown in FIG. 4b, i.e. if the switch is closed, a parasitic resistor 38 is present. The smaller a transistor 33 is, the larger the resistor 38 becomes and the smaller the capacitor 37 becomes. The larger resistor 38 is responsible for losses in the ON-state. The larger a transistor 33 is, the smaller the resistor 38 becomes and the larger the capacitor 37 becomes. The larger capacitor 37 is responsible for leakage in the OFF-state.
Parasitics become more severe as more signal power passes through the switch. In an ON-state, the losses thus become larger, and in an OFF-state, the leakage becomes larger.
The disadvantage with parasitics is clearly evident for the phase shifter using two branches between which a switch is placed. See for example the branch switch 12 and second branch switch 13 in FIG. 1a. These switches 12, 13 are placed where the entire signal that is to be phase shifted passes, and are called serial switches since they are coupled in series with the main signal path. Therefore, the disadvantage with parasitics in the switches is clearly apparent in this case.
For a phase shifter using reconfigurable filters, branch switches are not used, since there is only one branch. Instead, as shown in FIG. 3, there are serial switches 27, 28, 29, 30 within the reconfigurable filters, through which switches 27, 28, 29, 30 all the signal that is to be phase shifted passes. Therefore, the disadvantage with parasitics in the switches is clearly apparent in this case also.
In some applications, it is desirable to use a phase shifter as a narrow-banded filter during initial verification procedures. Therefore, it is of interest to present a phase shifter that, in a first mode of operation, has such a bandwidth that it does not affect the bandwidth of the system in which the phase shifter is used in a negative manner, and, in a second mode of operation, may be used as a narrow-banded filter.
The main disadvantage with the previously known phase shifters 1, 21 is the previously mentioned limited bandwidth of the phase shifter amplitude performance. Furthermore, a cascaded total phase shifter 20 inevitably results in an even narrower band where the phase shifting may work adequately, since the phase shifting elements, each having a narrow overlap band, do not have exactly the same narrow overlap band. This results in an even narrower total overlap band. In other words, as the resolution of the total phase shifter is increased, the bandwidth is decreased. As described above, limited bandwidth of the amplitude performance is not possible to compensate for by means of calibration methods.
There is thus a need for a phase shifter that, has such a bandwidth that it does not affect the bandwidth of a system in which the phase shifter is used.